Purified terephthalic acid (PTA) is a starting material for the formation of polyester resin, which is, in turn, used to make many materials of commerce having a variety of utilities. Purified terephthalic acid is formed from "crude" terephthalic acid conventionally by a number of purification methods, often with the aid of catalysts. The methods for purifying crude terephthalic acid heretofore available are not completely satisfactory either from an engineering standpoint, or from an economic standpoint, yet the purity of the purified terephthalic acid is an important determinant of the satisfactoriness of the processes by which the polyester resin is formed.
A number of reaction systems are known for forming crude terephthalic acid from a variety of starting materials. The purification aspects of the present invention may be used with substantially any of these reaction systems, but in accordance with the invention it is preferred that a reaction system involving the oxidation of paraxylene (p-xylene) be employed, and the use of such a synthesis system forms a part of the present invention.
The problems of existing and prior systems for producing purified terephthalic acid center around the difficulties in running the reaction systems to produce good yields of crude terephthalic acid economically, compounded by the difficulties of refining the crude terephthalic acid to eliminate impurities and unwanted components to produce purified terephthalic acid of a quality suitable as a starting material for producing polyester. Concomitant problems in prior systems include the high capital investment required for PTA plants, the severity of operating conditions of prior processes, both for the production of crude terephthalic acid, and for its purification, and the need for handling catalyst systems and reaction solvents, as well as reaction byproducts in a way such that environmental problems are minimized, and loss of material is also controlled.
One factor of importance in the production of purified terephthalic acid is the formation of crystals having a size and shape giving them good handling characteristics, washability and filterability.